"A flashy feature that has limited usability for victory."
Riff uses one of these in thisSluggy Freelance strip. At first a gatling gun that fires 100 stakes per second sounds like a great anti-vampire weapon. But when you realize that it can only hold one hundred stakes at a time and takes two days to load ... well, you can stake one vampire really, really good. The other dozen or so will tear you to pieces.
He eventually makes it better by adding a beltloader, similar to a mini-gun.
Most of Riff's inventions (at least in the earlier days of the strip) seem to fall into this problem. Look at some of the later (and alternate universe), stuff, however...
Sword-Chucks were only recognized as unusable by every idiot in the cast. White Mage pointed out they could be useful with a longer chain.
Staff-Chucks: Like sword-chucks, except that they shoot magic in every direction. Black Mage loved them, since destroying everything in sight is his favorite thing to do.
Black Mage's Hadoken could count since, while it's incredibly powerful, he can only use it once a day.
Vaarsuvius of The Order of the Stick has chosen to specialise in evocation magic at the expense of conjuration and necromancy. In layman's terms, this means s/he has access to a lot of flashy destruction spells at the expense of losing access to many 'utility-effect' spells (teleport, summoning minions, level drain, etc). Many experienced D&D players are of the opinion that damage-dealing evocations are one of the least optimal ways of utilising a wizard's power. The method Vaarsuvius eventually found to counteract this temporarily - the Soul Splice - wasn't much better. seeing as it involved renting Vaarsuvius' soul (trust me, It Makes Sense in Context) and only lasted so long, as well as making him/her feel sufficiently omnipotent to take on Xykon and the rest of Team Evil... single-handedly. In short, once is enough.
The upside is that after the White Queen's plan succeeds and PM puts on her ring, Jack Noir is effectively neutralized since he is no longer a hundred times more powerful than everyone else. Jade also gets First Guardian powers by merging with Jadesprite while ascending to god tier.
Except it goes right back to being a problem when Jade is mind-controlled by the Condescencion upon entering the Alpha session, allowing her to take control of Jane through the tiaratop and zapping Dirk, the person most likely to beat her, to outside the universe.
The Unreal Air gives you unlimited vertical flight. Stable, blandly slow, unsteerable flight. Dave finds a good enough use for it to ascend to his Gates, but it has no other practical use.
A Loonatic's Tale: Lynch Cruor fights using a Punjab lasso; in layman's terms, a weaponized hangman's noose. Unfortunately, he's a vampire who's just shy of 1000 years old, so he's had plenty of time to practice.
Yehuda Moon And The Kickstand Cyclery has Arboritum, an extremely light alloy that would be excellent for bikes if it wasn't also extremely fragile, ensuring the rider serious injury from frame cracking.
In Misfile, Vashiel is an angel of punishment, so he gets a Big Fucking Sword that flames and can smite evildoers in a second. His brother, Rumisiel, is a filer, so he gets a multi-filing tool, with a recorder, web access, and other Boring, but Practical things like that. Guess which one comes in handy?
Invariably, most Sparks use this as their modus operandi in Girl Genius, with many being disposed of by their own faulty/crazy creations.